Focal plane arrays typically consist of two dimensional arrays of individual and separate detectors—or pixels—which must be physically isolated from one another (mesas) in order to be able to generate an image of a scene devoid of any crosstalk or signal admixture between adjacent pixels. The detectors themselves are commonly made of an appropriate semiconductor material, regions of which can be doped either p-type or n-type. The junction between regions of opposite doping characters forms a so-called p-n photodiode which has the important ability to generate an electrical current when exposed to the appropriate illumination. While the doping can be created in a two dimensional pattern of islands in bulk material by techniques such as impurity diffusion or ion implantation, modern focal plane arrays are commonly fabricated by methods such as molecular beam epitaxy (MBE) or chemical vapor deposition (CVD). In MBE and CVD, semiconductor layers of different doping types are epitaxially grown sequentially in situ. In this case, the p-n junction is buried at some depth below the surface of the wafer down the sequence of grown layers over the entire substrate. Subsequent processing demands that trenches be etched in a two-dimensional mesa pattern in order to define individual pixels. Indeed, vertical mesa walls can be several microns deep. There are significant drawbacks to the requirement that trenches be etched in a two-dimensional mesa pattern in order to define individual pixels. First, the fabrication becomes significantly more complicated. Second, the etching exposes the p-n junction, introducing the possibility of surface leakage current, which often requires some passivation technique to mitigate. U.S. patents describing photodetector design approaches include U.S. Pat. Nos. 7,687,871, 7,737,411, 7,795,640, 7,928,473, 8,003,434, 8,004,012, and U.S. Pat. No. 8,274,096. Papers describing effects include Chanh Nguyen, Berinder Brar, Herbert Kroemer, and John H, English, Surface donor contribution to electron sheet concentrations in not intentionally doped InAsAlSb quantum wells, Applied Physics Letters, vol. 60, No. 15, 13 Apr. 1992, pages 1854-1856 and Chanh Nguyen, Berinder Brar, Vijay Jayaraman, Axel Lorke, and Herbert Kroemer, Magnetotransport in lateral periodic potentials formed by surface layer induced modulation in InAsAlSb quantum wells, vol. 63, No. 16, 10 Oct. 1993, pages 2251-2253. All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
What is needed is a focal plane array that is simpler to fabricate and that mitigates surface current leakage without passivation.